Bicycle magazine back in the 80s had an article about these sort of designs. I remember one where the frame was reduced to a single triangle by having the cranks each linked to the rear axle by a short chain. It wasn't direct drive, there was some gearing up - as you'd need, even on a track bike. The rider was leaning so far forward, there was an 'air-cooled saddle'. This consisted of two leather straps. The rider's wedding tackle sort of dangled in the breeze through the slot between them.
Not long after that, the idea of concentric rear wheel and bottom bracket axles popped up again in the magazine. This made it possible for a bike to have a single frame triangle, with the rear wheel tucked in underneath the rider, and have a freewheel. There was also a very short wheelbase tandem. The stokers seat was effectively cantilevered out behind the rear wheel of what was otherwise pretty close to the geometry of a normal bike, and the stoker's cranks concentric with the rear wheel axle.
Another detail I remember is that the magazine's Italian correspondent took such a tandem out for a spin, borrowing an attractive lady stoker, Ornella, for the purpose.
